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Carbs and Fuel System
From the gas tank to the carbs
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post #1 of 7 (permalink) Old 10-15-2011, 07:32 AM Thread Starter
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Help!

I think my carbs are way out of sync. the front plugs look good, maybe a little rich but still mostly brown. My rear plugs are fouled and wet. 142.40 jets front and rear, 3 turns on mix screws. bike seems to idle on only the front jug but runs great after about 2000 rpm. i tried the hose, yardstick snc tool and couldnt get it to work. anyone in the maryland area (baltimore, ft meade) ytha has a sync tool that i could meet up with? a "friend" rebuilt my carbs for me...came over to find them seperated and not sure if he messed with the sync.
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post #2 of 7 (permalink) Old 10-15-2011, 12:29 PM
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Could you possibly have an ignition problem?
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post #3 of 7 (permalink) Old 10-15-2011, 07:49 PM
If only it had 6th gear..
 
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you've most likely got a stuck float. After 2500 rpms it'll drink enough fuel to start to clear out but at idle your float is allowing gas to pour in and flood everything

Dave

Mods:
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Converted ACCT's to manual (thanks wib714)
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post #4 of 7 (permalink) Old 10-15-2011, 07:59 PM Thread Starter
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a can of seafoam and a hard ride...seems to be cleared out now.
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post #5 of 7 (permalink) Old 10-16-2011, 05:29 AM
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I'd say it was more likely you had a partially plugged idle jet in the rear carb. Idle jets plug up easier, and are more difficult to get cleaned out without actually physically cleaning them. The main jet is used most of the time, with lots of fuel flowing through it at highway speeds, so even if it gets slightly plugged up, highway riding for a few miles will usually clean it out. But the idle jet only gets fuel flow during low rpm use, and often doesn't get enough fuel flow through it to keep it clean. Constantly using Seafoam will usually work, but it's cheaper to drive a car. The main cause of plugged jets is either/both sitting around too much, and very poor quality gas. There was a time when gasoline (real gas, not moonshine) lasted longer than it does today.

The VN750 has a vacuum operated petcock, but I have seen carbs on bikes that didn't get completely gummed up it looked like they were full of Jello after 2-3 months, even though the gas in the tank was still good, or at least usable. The petcock was left on, and the float bowl on a carb is fairly small, and vented. Gas evaporates out of them quick. And with an open line to the tank, as gas evaporated, the needle valve let more in, and it too evaporated, all the while leaving behind the goo that does not evaporate. Pretty soon there was so much of that there was no more room for gas. I have cleaned many carbs in this condition. Over time this is inevitable, but it was amazing just how fast it happened. Again, just poor quality gas.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
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post #6 of 7 (permalink) Old 10-16-2011, 08:06 PM
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Quote:
Originally Posted by VN750Rider/Jerry View Post
I'd say it was more likely you had a partially plugged idle jet in the rear carb. Idle jets plug up easier, and are more difficult to get cleaned out without actually physically cleaning them. The main jet is used most of the time, with lots of fuel flowing through it at highway speeds, so even if it gets slightly plugged up, highway riding for a few miles will usually clean it out. But the idle jet only gets fuel flow during low rpm use, and often doesn't get enough fuel flow through it to keep it clean.
I know you are a licenced and experienced mechanic Jerry, but I disagree about the idle jet only working at low RPM on the vn750. This topic was discussed earlier this year on the board as many others were confused about the idle/pilot jet only working up to 3-4k rpm. I don`t recall who explained it, but it was simple and makes sense to me.

As I recall it comes down to a couple of plain and simple facts.
The pilot jet doesn`t have any valve or anything to shut it off.
It sits in the same little reservoir of gas as does the main jet, and continues to flow fuel at all speeds.
It doesn`t contribute much fuel volume in comparison to the main jet, but it does continue to get a fuel flow at all speeds.

I do agree that because of the idle/pilot jet`s small diameter opening (of ~0.040"+/- a few thousandths) that it is more susceptible to clogging up with gum or varnish left over from fuel evaporation in the carbs when sitting for long periods.

I come here to learn more, so if you can explain how or why a clean idle jet doesn`t have fuel flowing through it at higher rpms, I will be happy to read and consider it.
However at this time I consider it a given that the idle jet has to continue to have gas drawn through it at all speeds.

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

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TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
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post #7 of 7 (permalink) Old 10-17-2011, 07:36 PM
If only it had 6th gear..
 
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I thought it was a float issue and still lean that way because of the soaked rear plugs. My pilots were clogged solid from this high quality gas that we pay thru the nose for. I had a float problem as well and that caused major flooding requiring an oil change before even cranking it over after the repair but the pilots being clogged just caused it simply not to idle, or more accurately, to die below mid 2k rpm. I too believe there's always something more to learn so, my educated guess is that too much fuel causing fouled plugs is logically A) float valve not closing when the bowl is satisfied or B) petcock diaphragm letting fuel in through the vacuum port while running low speed. l'm always open to hearing other theories though . Peace.

Dave

Mods:
Jetted, shaved & coastered
Home made under-tank single filter
Converted ACCT's to manual (thanks wib714)
Dual vac lines on petcock (my cure for ticking)
LED tail/plate lights
Relocated helmet lock to license plate

Added stuff:
Customized hard bags w/ lights
Chrome levers
Hellcat windscreen
LED running lights
Home made 'Vita-Volt' meter
Foot rests on engine guard
Tunes

Hmm....she was on the back when we left.... ??

Last edited by DavesVulster; 10-17-2011 at 08:12 PM.
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